The present invention relates to digital X-ray radiography and can be used in screening of baggage, hand luggage and other objects in the course of customs and security inspection, as well as in medical diagnostics of early stages of various diseases by separate visualization of different tissues and organs, including medical tomography.
The main task of customs inspection screening is reliable detection and recognition in luggage, cargos etc. of hidden materials and objects that are forbidden for transportation (drugs, poisons, explosives, inflammable substances; fissionable materials hidden inside radiation-proof containers; cold and fire arms, precious metals, various kinds of smuggled objects, etc.). An important task of modern medical radiography and tomography is reliable recognition of pathologies in various organs and tissues allowing diagnostics of dangerous diseases, especially at early stages (initial tumor formation on the background of healthy tissue, soft blood plaques in arteries at early stages of atherosclerosis, etc.).
An X-ray method of luggage content recognition is known [Bekeshko N. A., Kovalev A. V. Radiation systems of luggage inspection. Zarubezhnaya Elektronika, 1988, No. 6, p. 2.], comprising raying of luggage that is moved on a conveyor belt by a lateral fan-shaped beam of X-ray radiation, recording of radiation that passed through the inspected object by a receiver, storage of the recorded signals and their reproduction in the form of TV image of the inspected luggage and inclusions.
This method allows recognition of luggage inclusions by their shadow X-ray image, i.e., distinguishing inspected objects by radiographic density of their images. However, the substance type of which said inclusions are made is not determined. Therefore, this method cannot detect forbidden substances (e.g., drugs or explosives) and distinguish them from ordinary organic substances—tea, coffee, sugar, beverages, etc.
A method is known of radiographic recognition of materials inside inspected objects, in particular, inclusions of specified chemical composition [Invention certificate of USSR No. 1583806, G01N 23/04], which comprises X-raying of the object, recording of the transmitted radiation in two different spectral ranges with different effective energies by one and the same radiation receiver, and comparing the ratio of logarithms of the recorded signals to a pre-set threshold value for subsequent decision on the type of the detected substance, e.g., whether it is organic or inorganic.
The device for realization of this method comprises: an X-ray source, a block for scanning of the controlled object, a one-dimensional matrix radiation receiver (detector array), a multiplexer, an analog-to-digital converter with a normalizer at the output, a video memory, a control, memory and buffer memory blocks, a comparator, a logarithmator, a video control block, and a color encoding block. In this method, separation of the initial continuous radiation spectrum into two separate spectral ranges with different effective energy is realized by coordinated periodic variation of the anode voltage of the X-ray tube.
Calculating the logarithm ratio of signals received by each of the detectors of the one-dimensional matrix radiation receiver for different energies, a mass attenuation coefficient ratio is, in fact, roughly evaluated for the substance of the inspected object. This ratio depends upon the effective atomic number of the substance. Comparing it with a specified threshold level, decision can be taken on whether the substance belongs to organic (Z<10) or inorganic (Z>20) materials.
The main drawback of this method that it does not allow recognition of certain specified substances with effective atomic number Zeff among other substances with close values of Zeff. E.g., a TNT-type explosive with Z=7.15 and a common soap with Z=6.25 would not be distinguished by this method. Also, emission of X-ray radiation in two different spectral ranges by switch-over of anode voltages on the X-ray tube and radiation recording by one detector array requires precise synchronizing of the source and detection system, which is a very difficult technical problem leading to even lower accuracy of Zeff determination.
Further practical experience of leading producers has shown another variant to be more promising—detection of X-ray radiation at fixed tube voltage by not one, but two detector arrays separated by a metal filter cutting off the energy ranges from each other. Each detector array is best suited for radiation detection in a separate energy range. As a whole, the assembly of low-energy detectors (LED) and high-energy detectors (HED) has a characteristic “sandwich” design [Harrison R. M. Digital radiography—a reviev of detector design. Nucl. Instr. and Meth., 1991, Vol. A 310, p. 24-34.].
A device for material recognition is known [U.S. Pat. No. 6,445,765, G01N 23/083, A61B 6/00], comprising an X-ray source and radiation receiver based on two (LED and HED) detector arrays. LED uses as scintillator a material in which at least one element has atomic number between 30 and 40, specifically, ZnSe(Te) crystal, and HED uses ceramics of material containing at least one element with atomic number above 60, specifically, Gd2O2S-type ceramics doped with Ce, Pr or Tb. LED thickness is 0.2-1.0 mm, HED thickness is 1-2 mm. LED is located, with respect to the X-ray source, before HED and is separated from it by a filter.
The use of ZnSe(Te) crystals as detecting elements of LED, as well as the relative position of LED and HED separated by a filter had been known before [Invention certificate of USSR No. 1639272, G01T 1/202; Patent of Ukraine No. 44547, G01T 1/202]. Due to the presence of two arrays of specially chosen detectors, which selectively detect radiation in the low-energy and high-energy ranges, accuracy of atomic number determination was substantially improved. Devices of this type have found wide practical application.
However, the accuracy of materials recognition remains not sufficiently high, which does not exceed 60-70% in the best variants of such devices (produced by Rapiscan, Smiths Heimann, NPO Kommunar, etc.). This may be sufficient for distinguishing between organics and inorganics or recognition of, say, cold or fire arms inside the luggage. But it is not sufficient for reliable recognition of explosives on the background of organics, for which the required accuracy should be about 90-95%.
Another method of X-ray material recognition by its effective atomic number [Patent of Russian Federation No. 2095795, G01N 23/04] comprises X-raying of the inspected object and recording of the transmitted radiation in spectral regions with different effective energy by two radiation receivers located one after another with a separating filter. Among the recorded signals, those are singled out that correspond to radiation passed only through the background substance in the inspected object and through both said background substance and the identified substance in the same object. Using signals of radiation absorption in two spectral regions by the background substance (for object parts where there is no identified substance), a calibration curve is chosen with each point corresponding to intensity values of signals of higher and lower effective radiation energy passed through the aggregate of the reference background substance (with signal values equivalent to the initial background substance of the inspected object) and the inspected substance of different thickness. Then signal values of the calibration curve are compared with selected recorded signals of radiation absorption by the aggregate of background and identified substances for the object. When the calibration curve contains signals equal to the recorded signals, it is judged that atomic numbers of the respective substances should be equal. Calibration curves are determined in advance at the same parameters of the emitter and receiver as in X-raying of the inspected object.
This method accounts for the spectral composition of radiation and the presence of a “typical” background substance. This eliminates some disadvantages of other detection methods of materials placed on the background of each other (so-called “multi-layered format”) by introducing the effective atomic number of one of them. The term “background substance” means any substance or combination of substances on the background of which the identified substance is located. Authors of this method claim that it allows detection of forbidden substances (drugs, explosives, etc.) on the background of various materials inside inspected objects.
A device for its realization comprises: an X-ray emitter, means for movement of the inspected object, an X-ray radiation receiver composed of two detector arrays (for low and high energies, respectively) separated by a filter, random access memory, an analog to digital converter, a video monitoring device, registers, an address generator, read-only memory, a comparison circuit, a marker signal shaper, control buttons, and signaling means.
Disadvantages of this method, as of all the other prior art, consist in impossibility to distinguish accurately and reliably among detected substances with close values of atomic number and density, since the proposed calibration curves will be practically identical.
A general drawback of the known solutions using LED and HED is identification ambiguity of explosives and drugs on the background of other organic materials. The reason for this lies in broad diffuse spectra of X-ray emitters, leading to the high-energy spectral range being partially recorded by LED, and the low-energy range—by HED. Another drawback is relatively slow response of ZnSe(Te) crystals. At low after-glow level, their decay time is more than 100 μs. This imposes limitations upon the full number of channels (up to 800 elements in the receiving system) and, respectively, on the minimum pixel size, i.e., on the spatial resolution of the system. Therefore, crystals with such decay time are not suitable for medical tomography.
In the above-described analogs, substance recognition by its atomic number uses a known dependence of the X-ray absorption coefficient on Zeff, which is different in different ranges of the radiation energy spectrum. In the working range of inspection scanners and medical tomographs (commonly used tube voltage up to 160 kV, i.e., radiation energy 100-110 keV), predominant absorption mechanisms involve photoeffect and Compton scattering, with absorption coefficient proportional to Zeff raised to 3rd-5th power in the case of photo-effect. The same law applies also to absorption in the scintillator material. This necessarily makes LED partially sensitive to high energies, and, inversely, HED show some sensitivity to low energies. When both detectors are placed one after another (in a row), with a broad emitter spectrum, this interference is rather strong, which necessarily lowers the measurement accuracy. As a result, the detection probability for, e.g., explosives on the background of safe organics, does not exceed 60-70%.
For efficient recognition of materials with similar character of radiation absorption, i.e., with close values of effective atomic number and density, an important factor, alongside with higher accuracy, is a possibility of simultaneous quantitative reconstruction of several controlled parameters. These parameters, determined by physio-chemical composition of the inspected material, include effective atomic number, density, and partial composition (concentration) of simple elements (e.g., carbon, nitrogen and oxygen in the problem of detection of explosives) or simple components for a material that is a complex chemical substance or a mixture (alloy, suspension) of substances.
For full reconstruction of these parameters, quantitative methods are needed that would not be based on the widely used principle of comparing the detected signals to data bases obtained in advance by screening of a large set of reference materials. Also, the direct X-raying methods are to account for requirement of nearly ideal separation of two or more detected energies, which is very difficult to achieve in a standard “sandwich” design of detector arrays.
The last of the above-described analogs has been chosen as prototype.